Level-maintenance for television amplifiers



p 19, 4K. SCHLESINGER 2,173,497

LEVEL MAINTENANCE FOR TELEVISION AMPLIFIERS Original Filed Aug. 1936'3-Sheets-Sheet 1 p 1939. K, SCHLESINGER 2,173,497

LEVEL MAINTENANCE FOR TELEVISION AMPLIFIERS Original Filed Aug. 4, 19363 sheets sheet 2 LEVEL MAINTENANCE FOR TELEVISION AMPLIFIERS OriginalFiled Ailg- 4, 1935 s Sheets-Sheet 3 Jm/enfor E Patented Sept. 19, 1939UNITED STATES PATENT OFFICE LEVEL-MAINTENANCE FOR TELEVISION AMPLIFIERSSteglitz, Germany Application August 4,

Renewed June 6, 1939. In Germany August 5,

2 Claims.

In my earlier application, Ser. No. 65,399ffi1ed Feb. 24, 1936, there isdescribed a circuit which, following on condenser-coupled low-frequencyamplifiers, ensures that the potential peaks are always adjusted to anequal, absolute voltage. In the circuit set forth therein the negativepeak values were in this way maintained at a certain level. In the caseof direct connection with the control grid of cathode ray tubes, or uponthe modulation of a transmitter from the bottom towards the top, thesenegative values of the am plifier output potential correspond with theblack in the image (black control). The feature of the inventionconsisted in the use of a diode connected as a peak-voltage meter forthe purpose of solving the problem.

Fig. 1 shows one form of my invention applied to the output circuit ofan amplifier.

Fig. 2shows the defect in the amplitude oscilloram and the correctiondesired.

Figs. 3 to 6 illustrate alternative forms of my invention.

By applying this diode Hi to the output circuit of the amplifier inaccordance with Fig. 1 it is possible not only to solve the problem ofmaintaining the negative peaks at a constant level, but beyond thistosupply a freely selective bias by way of the lead I to the consumingapparatus 2, for example the wireless transmitter. In Fig. 1 inaccordance with the invention the bias passes from the battery 3 by Wayof the leak resistance 4 to the connecting lead 5, which is traversed bythe total of bias and alternating potential. The valve 6 is the finaltube of the image amplifier, and is coupled by condenser l 35 with thepreliminary stage it. The bias is impressed at the grid of tube 6 by abattery 8 through resistance II.

In practical operations making use of this circuit it is found that theload represented by the 40 consuming apparatus 2 prevents exactadjustment of the peak potentials to the desired value and imparts alaxity to the level-maintaining means.

In Fig. 2 there is shown the curve a which an amplitude oscillogramshould have in the case of a level-maintaining means acting in idealfashion. The potential of the battery 3 is plotted as a certain positivevalue with the same 50 figure in relation to the earth potential zero.The negative synchronisation impulses 8 should always reach with theirpeaks the line 3. If there is a load 2 present, its leakage currentcauses a discharge of the condenser 9, the elec- 55 trade of whichconnected with 2 thus suffers a 1936, Serial No. 94,130.

loss of positive bias. The rectifying charge subsequently supplied bythe diode ll] occurs in it self merely for such time as the potential oflead 5 drops at times below the potential of lead l. This is the case,however, only during the transmission of black points of the image.Frequently the synchronisation impulse i is the only substantiallynegative part of the curve, and in the case of a load 2 it is notsufficient to cover the potential loss which occurs duringa period. Inconsequence the curve a. in Fig. 2 passes into the curve b,corresponding to a'potential slip d. Maintenance to the desired level 3has been unsuccessful. In this way interferences in the synchronisationmay occur, and grey parts of the image maybecome deep black.

As a remedy against the slip in amplitude there are set forth thefollowing three methods in accordance with the invention:

1. To furnish the diode with a higher alternating potential.

2. To bias the diode.

3. To place the diode in a grid circuit where it is not loaded.

Fig. 3 illustrates the first method. If there is passed to the consumingapparatus 2 by way of a condenser 9 merely a fraction of the oscillationpotential, which has been adjusted at a tapping l2 of the anoderesistance of the final valve 6, and if the diode I0 is coupled by wayof a condenser l3 with the full oscillation potential of the finalstage, the diode produces in idle no-load- Work at the leak resistance 4a potential which is equal and opposite to the negative peak value ofthe oscillations amplitude. If the diode is now connected by way of theresistance It with the modulation lead 5 and by way of the lead I withthe bias 3, the said slip in amplitude by reason of the load at 2 iscompensated by the over-excitation of ID. 40

The second method is explained in Fig. 4. It avoids the waste ofoscillation amplitude involved by the method above described. In Fig. 4,there is connected in series to the diode an auxiliary battery I5, bymeans of which the initiation of the electronic current in It! isadvanced. By adjustment of IS the slip in amplitude may be exactlycompensated. The circuit is naturally also capable of being used in thecase of mains connections.

The third method of solving the problem which is to be particularlyrecommended consists in removing the diode l0 entirely from all loadedcircuits and disposing the same in the grid circuits of the poweramplifier. Two schemes of this kind are shown in Figs. 5 and 6. In Fig.5 there is shown a preliminary amplifier tube It, which is coupled byway of a condenser I in R,C connection with the grid of the final tube6. In this case the valve 6 must be provided with a special insulatedpotential source is for grid and anode. Since the sign of anoscillation, which is negative in the anode circuit, is positive in thegrid circuit, the polarity of the diode I!) must also be changed, and itis connected by the anode with the control grid of 6, and in parallel toits leak resistance H. The tube Hi operates without error in this form,even without over-excitation or auxiliary bias, so long as the tube isnot traversed by grid currents. This must be ensured by means of asufiiciently large grid bias H3. The cathode of the tube 6 is earthed byway of a condenser ill.

Fig. 6' shows a circuit in which the bias necessary for maintaining aconstant level is also taken from the grid circuit of the amplifier 6,but in which this bias itself is not conjointly amplified in the tube 6but is employed in direct fashion. The circuit, therefore, possessescertain disadvantages as compared with Fig. 5. The diode I9 is connectedbut in series with condensers 22 and 20, and accordingly loads the gridcircuit of 6 but to very little extent. The anode of 6 is connected tothe consuming apparatus 2 by Way of a condenser 9 and lead 5. Theregulating bias is received by this consuming apparatus from a condenser20 through the medium of a leak resistance 2 I. The condenser 20 ispositively charged by the tube NJ, as it is connected with its cathode.The anode of the tube is connected by way of a condenser 22 with theanode of the preliminary valve IS. The bias is only equal to the maximumout put potential of 6 when the amplification of E is equal to 1:1, asthe excitation of H3 takes place only with the grid alternatingpotential of 6. The tube l 6 is bridged by a leakage 23, whilst thedesired bias 3 is supplied by way of a resistance 24. Immediately thecondenser 2Q has been charged to a positive bias, which is equal to themaximum positive potential of the grid of 6, all flow of current to thecondenser 22 ceases, and a loading of I5 by the regulating elements nolonger takes place. A possible slip may be compensated also in this caseby the use of over-excitation or auxiliary potential. An independentcompensation of the slip, however, occurs automatically in this caseowing to the fact that the leak current flowing to the consumingapparatus 2 by way of the resistance 23 biases itself. The drop inpotential at 23 biases the anode of ill positively in relation to thecathode and facilitates or advances the initiation of the emission.

Finally it may be mentioned that the devices set forth in the above,which are employed here for maintaining the negative peak values at aconstant level, may also be employed without difiiculty for maintainingconstant peak values of positive sign merely by changing the polarity ofthe two-electrode tube it. This may be necessary in all transmissionmethods in which modulation is performed in reversed fashion, i. e.,with increasing aerial current upon decreasing light intensity. In Fig.5 there is shown a reversepolarity connection of this kind renderednecessary by the fact that the reversed polarity has required to beconsidered within an amplifying stage. Generally speaking, the sliperrors are considerably greater when maintaining positive peak values,as in all of these cases the bias of the modulating line 5 requires tobe raised to a high positive potential and the leak currents arecorrespondingly greater. Upon connecting the diodes in the grid circuitsin accordance with Fig. 5 these difficulties are overcome withouttrouble.

It may be specifically remarked that in the above there have beendescribed merely particular forms of embodiment of the general ideaaccording to the invention, and that the stated measures may be employedboth in combination as well as singly.

I claim:

1. In a television apparatus comprising an amplifier tube the grid ofwhich is supplied with the mixed image and synchronisation signals andthe alternating anode potential of which is conducted to the input of aconsuming apparatus, a device for maintaining constant the potentiallevel or" input independently of the consumption of said apparatus, saiddevice consisting of a diode shunting the grid leak of said amplifiertube thus ducing a bias equal to the amplitude of said al-..rnatingpotential.

2. In a television apparatus comprising an amplifier tube the grid ofwhich is supplied with the mixed image and synchronisation signals andthe alternating anode potential of which is conducted. to the input of aconsuming apparatus, a device for maintaining constant the potentiallevel of said input independently of the consumption of said apparatus,said device consisting of a diode which is connected on one side via acondenser to said grid and via a resistance and a bias source to earth,and on the other side viaa resistance to said input and via acondenserto earth, and which is shunted by a high resistance, said amplifierhaving an amplification factor of about 1 so that the bias produced bysaid anode is equal to the amplitude of said alternating potential.

KURT SCI-ILESINGER.

